Electrostatic spray coating is used for the deposition of coating materials upon a workpiece and electrostatic spraying enhances the amount of coating material received on the workpiece by means of the electrostatic field between the spray gun and the workpiece. This electrostatic field is established at or adjacent to the atomizing outlet of the spray gun whether the coating material is pneumatically or hydraulically atomized. The electrostatic potential is normally generated with a conventionally produced direct current source of between 30 and 150 kilovolts. The most usual working voltage for hand-held spray guns is in the 50-60 kilovolt range, so that the generally desirable minimum gradient of 5 kilovolts per inch can be established between the high voltage charging area and the object being sprayed, with a normal 10-inch separation between the head of the gun and the workpiece. This is described in the Juvinall et al. U.S. Pat. No. 3,048,498, and produces a direct current output with minimum AC ripple in the output, less than 10% ripple.
It has been recognized that higher charging voltages generally increase the electrostatic attractive force. The ability to spray uniformly a cylindrical object from a single lateral direction is a measure of the "wrap" efficiency and is indicative of the magnitude of electrostatic force and DC voltage.
The prior art has disclosed three basic systems for producing electrostatic potential for electrostatic spray coating of material. The oldest is the use of a conventional high voltage transformer, energized at commercial frequencies, e.g., 60 Hz, supplying a half or full-wave rectifier. This is a fixed unit and supplies the high voltage output, commonly 55 kilovolts DC, by means of a coaxial cable to the spray gun. The second known system is the electrogasdynamic system in which the power supply output is physically smaller and has a very low power supplying an output of about 5 kilovolts, which potential is carried to the spray gun by a coaxial cable, and this potential is used to generate a cumulative charging of a supersonic column of alcohol-laden air, which at its output creates a DC potential of 55 kilovolts or higher, depending upon several variable factors. This system is illustrated in the Cowan U.S. Pat. Nos. 3,651,354 and 3,791,579. Like the first system, it requires a separate power supply and electric cable from such power supply to the spray gun.
The third prior art system is illustrated in the Malcolm U.S. Pat. No. 4,219,865, which dispenses entirely with an electrical cable connected to the spray gun and, instead, utilizes six miniaturized components within the spray gun, with components to achieve the high voltage in six steps. These six components include an alternator, rectifier, oscillator, transformer, and a voltage multiplier. The turbine is an air-driven turbine driving an alternator producing about 15 volts, which is rectified, and then this operates an oscillator operating at about 20 kilohertz at 12 volts. The oscillator has a square wave output which can be multiplied in a toroidal transformer to a value of about 2500 volts. This, in turn, is multiplied by a conventional cascade halfwave voltage multiplier of about 20 stages to produce a normal 50-55 kilovolt output. The cascade multiplier is a half-wave rectifier, and this oscillator-to-multiplier system is designed to produce the 55 kilovolts as a DC voltage with a minimum of ripple voltage or peaks because the square wave input being rectified makes a practically constant DC output. This third system produces spray painting results which are generally equivalent to the Cowan second prior art system or the system shown in the Juvinall patent.
In all these three prior art systems, the objective is a uniformly charged paint particle, charged at or about the uniform DC voltage output generated by the system. As may be observed from the teachings of the prior art systems, the first and second systems are burdened with the objectionable electric cable, which may be stiff and bulky, and can hamper the operation of the spray gun. The third system has a rather complex sequence of five electrical components, i.e., the alternator, the rectifier, the oscillator, the toroidal transformer, and associated electronic regulating devices needed to convert the simple low voltage of about 12 volts AC to a controlled level sufficiently high to provide a minimum input to the series voltage multiplier. It has been observed that the circuitry just described, necessary for the conversion of low voltage, low frequency, e.g., 250 Hz at 12 volts, into high frequency and higher voltage, e.g., 20 KHz at 2500 volts, is subject to overheating and breakdown of the components when they have been miniaturized sufficiently for installation in a hand gun.
This third system is designed for a uniform square wave output from the oscillator so that when run through the series voltage multiplier, it is a DC output free from excessive ripple or peaks. Currently manufactured systems of this third type have been prone to premature failure under constant duty, as distinguished from intermittent duty.
The problem to be solved, therefore, is how to construct a spray gun apparatus which may be hand-manipulable and which has small, lightweight components so that the spray gun is not burdened by being connected by means of an electrical cable to any external apparatus, yet a high voltage is established with safety to the spray gun operator and which has high "wrap" efficiency.